Polyester fibers have been produced in the past under a variety of melt spinning conditions. Both high stress and low stress spinning processes have been employed. Under high stress conditions the as-spun filamentary material is withdrawn from the spinneret under conditions whereby substantial orientation is imparted to the same soon after it is extruded and prior to its complete solidification. See, for instance, U.S. Pat. Nos. 2,604,667 and 2,604,689. Such high stress conditions of the prior art commonly yield a non-uniform filamentary material having an internal structure wherein substantial radial non-homogeneity exists across the fiber diameter leading to self-crimping characteristics upon heating, or less than desired tensile properties.
Polyester spinning processes have also been proposed wherein the cooling of the extruded filamentary material has been retarded (i.e., prolonged) prior to complete solidification so as to alter the properties thereof. See, for instance, U.S. Pat. Nos. 2,323,383; 3,053,611, and 3,361,859.
Heretofore, polyester fibers following extrusion and solidification commonly have been drawn while at an elevated temperature to further enhance their tensile properties. Such drawing may be conducted in an in-line fashion following fiber formation or after the as-spun fiber is unwound from an intermediate collection device. Such drawing is commonly conducted upon contact with an appropriate heating device, heated gaseous atmosphere, or heated liquid medium. Also, it has been known that previously drawn polyester fibers may be heat treated with or without allowed shrinkage (i.e., post-annealed) in order to modify their physical properties.
As-spun polyester filamentary material consisting principally of polyethylene terephthalate, because of its extremely slow crystallization rate at room temperature, forms a stable fiber package unlike an as-spun polyamide filamentary material. As-spun polyamide filamentary materials have a marked tendency to rapidly crystallize at room temperature with an accompanying growth in fiber length thereby rendering wound fiber packages of the same highly unstable and difficult to handle. See, for instance, U.S. Pat. No. 3,291,880 which discloses a process for treating an as-spun polyamide yarn with steam so as to render it capable of forming a stable fiber package. A comparable treatment of an as-spun polyester filamentary material has been completely omitted, since the need for such intermediate processing is absent. Also a polyamide filamentary material commonly is taken up following melt extrusion and solidification at a lower stress for a given take-up speed than a polyester filamentary material formed using the same equipment because of the varying extensional viscosities of the polymeric materials.
While the prior art has been capable of producing polyester filaments suitable for use in commercial applications, no polyester filament is known to have been heretofore produced having the internal structure and resulting property balance of the polyester filament which forms the subject matter of the present invention.
It is an object of the present invention to provide an improved polyester filament possessing a unique microstructure.
It is an object of the present invention to provide an improved polyester filament suitable for use in commercial applications.
It is another object of the present invention to provide an improved polyester filament exhibiting a balance of properties heretofore never achieved in prior polyester filaments.
These and other objects, as well as the scope, nature and utilization of the invention, will be apparent to those skilled in the art from the following description and appended claims.